1. Field of the Invention
The present invention relates to a driving device, a display apparatus using the driving device, and a driving method for the display apparatus and, more particularly, to a driving device for driving a current-driven optical element, a display apparatus for driving a simple matrix type display panel having display elements formed from a current-driven optical elements by using the driving device, and a driving method for the display apparatus.
2. Description of the Related Art
The recent years have seen a considerable proliferation of display apparatuses and devices, such as liquid crystal displays (LCDs), replacing cathode-ray tubes (CRTs), as the monitors and displays of personal computers and video equipment. Liquid crystal displays, in particular, have quickly come into widespread use because they can achieve decreases in thickness and weight, space saving, a reduction in power consumption, and the like as compared with conventional display apparatuses (CRTs). In addition, relatively small liquid crystal display apparatuses have been widely used as display devices for cell phones, digital cameras, personal digital assistants (PDAs), and the like which have recently become considerably popularized.
The following are expected as next-generation display devices (displays) and display elements following such liquid crystal displays: organic electroluminescence elements (to be abbreviated as “organic EL elements” hereinafter), inorganic electroluminescence elements (to be abbreviated as “inorganic EL elements” hereinafter), and display devices having spontaneous emission type optical elements such as light-emitting diodes (LEDs).
Among the above display devices having various kinds of spontaneous emission type display elements, display devices having display elements formed from organic EL elements made of organic compounds as light-emitting materials have recently undergone vigorous research and development toward practical application and commercialization because technical achievements superior to those obtained in other kinds of display elements have been obtained in terms of color display, low-voltage drive techniques, and the like.
FIGS. 13A, 13B, and 13C respectively show the schematic arrangement of an organic EL element, its voltage-current characteristic, and an equivalent circuit of the organic EL element. The structure, emission principle, and emission characteristics of the organic EL element will be briefly described below.
As shown in FIG. 13A, for example, an organic EL element OEL has an arrangement in which an anode electrode (positive electrode) 112 made of a transparent electrode material such as ITO (Indium Thin Oxide), an organic EL layer 113 made of a light-emitting material such as an organic compound, and a cathode electrode (negative electrode) 114 made of a metal material and having a reflection characteristic are sequentially stacked on one surface of a transparent insulating substrate 111 such as a glass substrate. The organic EL layer 113 is formed by, for example, stacking a hole transport layer 113a made of a polymer-based hole transport material and an electron transport light-emitting layer 113b made of a polymer-based electron transport light-emitting material.
In the organic EL element OEL, as shown FIG. 13A, when positive and negative voltages are applied from a DC voltage source VDC to the anode electrode 112 and cathode electrode 114, respectively, light hν is emitted on the basis of the energy produced when holes injected into the hole transport layer 113a recombine with electrons injected into the electron transport light-emitting layer 113b within the organic EL layer 113. For example, the light hν is transmitted through the anode electrode 112 and emerges from the other surface side (upper side in FIG. 13A) of the insulting substrate 111. In this case, the emission intensity (i.e., the emission luminance of the organic EL element) of the light hν is controlled in accordance with the amount of current flowing between the anode electrode 112 and the cathode electrode 114.
In this case, the voltage-current characteristic of an equivalent circuit of the organic EL element OEL exhibits a similar tendency to that of a diode, as shown in FIG. 13B, and the electrode layers (anode electrode 112 and cathode electrode 114) oppose each other through the relatively thin dielectric layer (organic EL layer 113). As shown in FIG. 13C, therefore, the optical element can be expressed as a parallel connection of a diode type light-emitting element Ep and a junction capacitance Cp. Note that the voltage-current characteristic of the organic EL element will be described in detail later in the embodiments of the present invention (to be described later).
As display driving methods for display apparatuses having display panels in which display elements (display pixels) having spontaneous emission type optical elements such as organic EL elements like those described above are arranged in the form of a matrix, the active matrix driving scheme and simple matrix (passive matrix) driving scheme are known. As is known, in the active matrix driving scheme, a selection switch and storage capacitance are provided for each display pixel to control the driven state (emission state) of each display element in accordance with the charge voltage of a corresponding one of the storage capacitances in the simple matrix driving scheme, the emission state of each display pixel is time-divisionally controlled by directly applying a predetermined pulse to the display element.
Although the active matrix driving scheme is superior to the passive one in terms of luminance and multi-gradation for image display, a pixel driving function such as a selection switch (thin-film transistor) must be provided for each display pixel. This complicates the apparatus arrangement and demands a more advanced micropatterning technique, resulting in an increase in product cost. In contrast to this, in the simple matrix driving scheme, there is no need to prepare a pixel driving function such as a selection switch for each display pixel, and hence the apparatus arrangement can be simplified. This makes it possible to improve the manufacturing yield and reduce the product cost.
The schematic arrangement of a display apparatus based on the simple matrix driving scheme will be described below.
FIG. 14 shows an example of the display apparatus based on the simple matrix driving scheme.
As shown in FIG. 14, the display apparatus based on the simple matrix driving scheme is roughly comprised of a display panel 110P having a plurality of scanning lines SL extended in a row direction, a plurality of signal lines DL extended in a column direction to intersect the scanning lines SL at right angles, and display elements (organic EL elements) OEL each formed near the intersection of the scanning line SL and the signal line DL. The apparatus further includes a scanning driver 120P which applies a scanning signal to each scanning line SL at a predetermined timing to sequentially scan the organic EL elements OEL on each row in the selected state, a data driver 130P which generates a driving current corresponding to display data and supplies the current to each organic EL element OEL through a corresponding one of the signal lines DL in synchronism with scanning by the scanning driver 120P, and a controller 140P which generates a scanning control signal, data control signal, and display data which are used to display desired image information on the display panel 110P, and supplies them to the scanning driver 120P and data driver 130P.
As driving methods for the display apparatus having the above arrangement, the following two methods are known. One method is a current designation type driving method in which the scanning driver 120P sequentially applies a scanning signal for selecting one of the scanning lines SL to the scanning line SL of each row on the basis of a scanning control signal supplied from the controller 140P in each predetermined scanning period, and the data driver 130P generates a driving current having a predetermined current value corresponding to display data in the scanning period on the basis of a data control signal and display data supplied from the controller 140P in synchronism with this scanning signal, and simultaneously supplies driving currents through the respective signal lines DL. Thus the respective organic EL elements OEL on a selected row emit light with a predetermined luminance level. The other method is a pulse width modulation type driving method in which the data driver 130P generates a driving current formed from a constant current value and having a signal time width (pulse signal width) corresponding to display data, and supplies the current to each signal line DL. Thus the respective organic EL elements OEL on a selected row emit light with a predetermined luminance level. This operation is sequentially repeated for each row corresponding to one frame on the display panel to display desired image information on the display panel 110P.
In the simple matrix driving scheme, a voltage driving scheme of driving each display element by applying a predetermined voltage from the data driver to the display element is known in addition to the above current driving scheme. Assume that the organic EL element is used as a display element. In this case, since each element has an arrangement in which the diode type light-emitting element Ep and junction capacitance Cp are connected in parallel as shown in FIG. 14, and each organic EL element OEL is connected in parallel with the signal line DL, the total sum of junction capacitances becomes large, and the interconnection capacitance of each signal line is added. As a consequence, in the voltage driving scheme, a delay occurs in the driven state of each display element or a voltage drop occurs in accordance with the distance from the data driver, resulting in, for example, variations in emission state (luminance) in the upper and lower areas of the display panel. This leads to a deterioration in display image quality. In a display apparatus using organic EL elements as display elements, therefore, the current driving scheme is regarded superior to the voltage driving scheme.
The display apparatus based on the above simple matrix driving scheme, however, has the following problems.
In the current driving scheme, operating a display element with a predetermined luminance level by supplying a predetermined driving current to it is equivalent to charging the junction capacitance or the like of a given display element with a driving current and also charging the junction capacitance of the remaining unselected display elements on a signal line to which the given display element is connected. In this case, as compared with the voltage driving scheme, a deterioration in response characteristic or the occurrence of variations in emission luminance can be suppressed by supplying a driving current having a large current value. Assume, however, that the driving current supplied from the data driver is set to a relatively small current value for the sake of the specifications of a power supply or power saving, or the total sum of the junction capacitances of display elements increases as the number of scanning lines increases and the number of display pixels increases along with increases in the size and resolution of a display panel. In this case, when the driving current is supplied to the display element at a driving timing, the response characteristics with respect to current and voltage values deteriorate, and the time required for a voltage applied to the display element to reach a predetermined value is prolonged, resulting in a noticeable lack of emission luminance and occurrence of variations.
FIG. 15A shows a change in supply current over time when a driving current is supplied to a display element. FIG. 15B shows a change in voltage applied to a display element over time. Referring to FIG. 15A, the abscissa represents the time; and, the ordinate, the supply current to the display element. Reference symbol Tspy denotes a supply period of a driving current; and Tdly, a delay time from the start of supply of the driving current to the start of operation of the display element. Referring to FIG. 15B, the abscissa represents the time; and the ordinate, the voltage applied to a display pixel in the forward direction. Reference symbol Vth denotes a threshold voltage for operation in the display element. As shown in FIGS. 15A and 15B, the rise characteristics of a current value and voltage value supplied to the display element deteriorate owing to the junction capacitance of the display element and the interconnection capacitance of a signal line. In addition, owing to variations in junction capacitance among the respective display elements, differences in interconnection capacitance among signal lines, and the like, the degree of deterioration varies. As a consequence, the amount of electric charges supplied to the display element in a driving current supply period decreases below the amount required for display with a desired luminance level, resulting in a lack of emission luminance or variations in emission luminance among the display elements. This leads to a deterioration in display state.